1. Field of the Invention
This invention relates to a two-dimensional optical scanner adapted to display a two-dimensional image by causing a beam of light that is scanning in a direction to also scan in a second direction. An optical scanner according to the invention can find various applications for displaying two-dimensional images. The present invention also relates to a method of driving such a two-dimensional optical scanner.
2. Related Background Art
(1) Two-dimensional optical scanners adapted to cause a beam of light to scan by means of an optical deflector and display an image have been and are being popularly utilized.
(2) FIG. 10 of the accompanying drawings schematically illustrates the structure of a known two-dimensional optical scanner of the type under consideration. The optical scanner comprises a light source 1 provided with a modulation means, a first and second optical deflectors A, B for deflecting a beam of light emitted from the light source and an emission correcting optical system 40. It is adapted to display a two-dimensional image by causing a beam of light that is scanning in x-direction to also scan in y-direction.
(3) Galvano-mirrors or similar devices that can temporally change the rate of angular displacement are normally used for the optical deflectors A, B. Such an optical deflector will be discussed in greater detail below.
Normally, a galvano-mirror is used for the optical deflector, which is driven by a saw-tooth wave signal as shown in FIG. 11A so that the displacement of the galvano-mirror may change linearly relative to the scanning time. Alternatively, the optical deflector may be driven by a sinusoidal signal as shown in FIG. 11B. Then, the rate of angular displacement of the optical deflector also changes with time.
In recent years, micro-optical deflectors (micro-mirrors) having a mirror angle of several millimeters have been realized (Japanese Patent Registration Publication No. 02722314) by utilizing techniques for processing silicon that is a semiconductor. FIG. 12 of the accompanying drawings is a schematic plan view of such an optical deflector (galvano-mirror), illustrating its structure. Referring to FIG. 12, there are shown a silicon substrate 50, an upper glass substrate 51, a lower glass substrate 52, a movable plate 53, a torsion bar 54, a planar coil 55, a total reflection mirror 56, an electrode terminal 57 and permanent magnets 60 through 63. The optical deflector is of the electro-magnetic type. A drive current is made to flow to the planar coil 55 so as to drive the device by means of the permanent magnets and Lorenz force (as will be described in greater detail hereinafter). A number of micro-mirrors of the electrostatic type and those of the piezoelectric type have also been proposed to date.
The illustrated optical deflector, however, can be driven only by a sinusoidal signal because it is operated by utilizing the resonance of the mirror at or near the resonance frequency of the latter. When a pair of galvano-mirrors or resonant optical deflectors are driven by a sinusoidal signal to display a two-dimensional image by way of raster scan, the scan speed of the light beam is high at and near the center of the image but that of one of the optical deflectors is reduced to nil in a peripheral area of the image and eventually the sense of speed becomes inverted. FIGS. 13A to 13C show the scanning characteristics of such an arrangement. For the above described reason, only a central area of the projection surface where the scan speed of the light beam can be held substantially to a constant level if corrected by an optical system is used for displaying images. In other words, the remaining peripheral area of the surface is not used for image display and hence it is an unnecessary area. Additionally, a galvano-mirror or a resonant optical deflector that is driven by a sinusoidal signal is so designed that the scan time in a positive direction is made equal to the one in the opposite direction. Therefore, when such a device is used to display an ordinary television image or a computer image, only the positive scan direction or the opposite scan direction is used, or if the two scan directions are used, the image information is processed and rearranged.
Now, a one-dimensional optical scanner will be discussed below.
FIG. 14 of the accompanying drawings schematically illustrates an ordinary one-dimensional optical scanner that is a laser beam printer comprising a polygon mirror 70. Referring to FIG. 14, a mirror (synchronism detecting mirror) 71 is arranged in a peripheral area of the image region so that the scanning light beam may be reflected by it and the light beam reflected by the mirror 71 may be detected by a photo-detector 72 for detecting synchronism in order to detect the drive timing for each scan cycle. In FIG. 14, reference symbol 73 denotes a light source provided with a modulation means and reference symbol 74 denotes an emission correcting optical system, while reference symbol 75 denotes a photosensitive drum.
(4) Meanwhile, with an optical scanner of the above described type, it is necessary to monitor the drive cycle and the drive timing of the optical deflector. This will be discussed below.
In the case of a two-dimensional optical scanner, when the optical deflectors are not synchronized for operation, the image displayed by the scanner can become distorted, and in the worst instance, may appear as if it were incessantly drifting. Obviously, the quality of such an image is low. However, the drive cycle and the drive timing of each of the optical deflectors are not constant but can vary as a function of ambient temperature and other operating conditions. In other words, if the optical deflectors are synchronized for operation once, it does not necessarily mean that they operate synchronously since then. Therefore, the actual drive cycle and the actual drive timing of each of the optical deflectors need to be constantly monitored so as to make them operate in a satisfactorily synchronized manner. Additionally, the two-dimensional image needs to be displayed in the corrected region of the emission correcting optical system and hence the actual drive cycle and the actual drive timing of each of the optical deflectors need to be constantly monitored from this point of view.
(5) However, while a one-dimensional optical scanner scans with a constant angular velocity, the optical deflectors such as galvano-mirrors used in a two-dimensional optical scanner are of the type where the rate of angular displacement changes with time. Therefore, the detection technique that is used in the one-dimensional optical scanner is not enough for the two-dimensional optical scanner. Japanese Patent Publication No. 2657769 discloses methods of detecting the angle of displacement of a galvano-mirror. They include the following.
(1) a method that utilizes a detection coil (see FIGS. 15A and 15B)
(2) a method that utilizes light (see FIG. 16)
(3) a method that utilizes an electrostatic capacitance (see FIG. 17).
These proposed methods will be discussed below.
FIGS. 15A and 15B of the accompanying drawings illustrate in detail (particularly the displacement detecting section of) a micro-optical deflector (galvano-mirror) prepared by using the micro-mechanics technology. The galvano-mirror has a three-layered structure formed by an upper and lower glass substrates 51, 52 and a silicon substrate 50 that is sandwiched by the glass substrates as shown in FIG. 13B. A flat movable plate 53 is swingably supported by the silicon substrate 50 by way of a torsion bar 54. A total reflection mirror 56 is formed at the center of the upper surface of the movable plate 53 by means of an evaporation technique and adapted to reflect a laser beam. On the other hand, a planar coil 55 is formed along the periphery of the movable plate 53 and permanent magnets 60 through 63 are arranged along a pair of opposite edges of the movable plate 53 so that the movable plate 53 and the total reflection mirror 56 are driven to swing when a drive current is made to flow through the planar coil 55. Additionally, detection coils 81, 82 are arranged on the lower glass substrate 52 at the respective positions illustrated in FIG. 15A so that the angle of displacement of the movable plate 53 can be detected as the change in the voltage signal output as a function of the mutual inductance is detected in terms of differential.
On the other hand, the arrangement of FIG. 16 is adapted to apply a light beam 91 to the rear side of reflector 93 for the purpose of detecting the angle of displacement and receive the reflected light beam by means of a PSD 92. The angle of displacement of the reflector 93 is detected by referring to the light receiving position of the PSD 92. Finally, the arrangement of FIG. 17 comprises a pair of electrodes 100, 101 disposed at opposite ends of the rear side of reflector 93 and another pair of electrodes 102, 103 disposed respectively vis-à-vis the electrodes 100, 101 to produce capacitors C1, C2. The angle of displacement of the reflector 93 is detected by referring to the difference of the capacitances of the capacitors C1, C2.
However, the operation characteristics of the sensor are vital when the angle of displacement is to be detected by means of a detection coil (FIGS. 15A and 15B) or electrostatically (FIG. 17). In other words, an offset can occur due to changes in the temperature characteristics and/or the positional error that can be produced when assembling the substrates. Then, the operation characteristics of the sensor have to be observed in advance in order to accurately detect the angle of displacement. When, on the other hand, the angle of displacement is to be detected optically (FIG. 16), it is very difficult to regulate the relative positions of the detection light source 90, the mirror 93 and the PSD 92 because of the long light path. Therefore, the PSD 92 is required to have a high positional resolution. Furthermore, with any of the above described known methods, it is not possible to accurately monitor the condition of the scanning light beam that is affected by the possible distortion and/or shift from the axis of revolution of the mirror because the methods are not adapted to directly detect the scanning condition of the light beam. Thus, it is highly difficult for an image display to display a high quality image only by using any of the above detection methods.
Therefore, it is the object of the present invention to provide a two-dimensional optical scanner that can ensure the monitoring operation of the photo-detector and effectively prevent degradation of the displayed image and a method of driving such an optical scanner.
The present invention is made in view of the above identified circumstances. According to the invention, there is provided a two-dimensional optical scanner adapted to display a two-dimensional image by causing a light beam scanning in a first direction to scan also in a second direction;
at least the scan speed of the light beam in said first direction changes in each scan cycle;
a first photo-detector for detecting the scanning light beam being arranged in a region where the scan speed of the light beam in said first direction is substantially equal to the highest speed thereof.